The reason behind unhooking her bra was it'll display her boobs infront of Kevin fully. Her boobs were getting out from her bra like almost. If you look little down you can clearly see her nipples.

She covered her back with her hair only to hide her unzipped dress and unhooked bra. She looked at her in the mirror ,the view was hot. Stella praised herself .Then she heard footsteps coming towards kitchen. She backed away from the mirror and stood on her previous place where Kevin pinned ...